Frequency detecting network



Feb. 26, 1946.1 H. M, STQLLEB l' Y 2,395,515

FREQUENCY DETECTING NETWORK /Nl/ENTOR H M. STOLLER ATTORNEY Feby26,1946 H.,M. s'roLLER 2,395,515

FREQUENCY DETECTING NETWORK Filed Nov. 2l, 1942 2 Sheets-Sheet 2 A L /NVEA/ron l By H MsoL/ ER AT TORNE V Patented Feb. 26, 1946 FREQUENCY nsrsc'rmo Na'rwoax Hugh M. Stoller, Mountain Lakes, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 21, 1942, Serial No. 466.507

5 Claims. l (CL T11-119) regulator systems for the purpose of detecting changes in the frequency of an alternating wave relative to a certain frequency to control the electrcal energy supplied to an electrical rnctor whereby its speed could be regulated. Such bridge networks were of the Wheatstone bridge type embodying four physical arms, one of which 'comprised an electrical winding and capacity tuned to the certain frequency. Certain prior art circuits disclosing such bridge networks in speed regulator systems are disclosed, for example, in

my Patents Nos. 1,695,035 and 1,711,661 granted December 11, 1928 and May '7, 1929, respectively.

These systems were used in cases where the factors of bulk, weight and ambient temperature variations were of no special concern. The present invention is particularly applicable to situations where the factors of space, weight and ambient temperature variations are of paramount importance, such as in the field of mobile apparatus.

The present invention contemplates a frequency discriminating bridge network adapted to detect variations in the frequency of an electrical wave with respect to a certain frequency; to provide an increased output for a given percentage variation in frequency; to accomplish the detection of such frequency discrimination with compensation for variations of ambient temperature; and to provide such bridge network with substantially minimum bulk and weight characteristics.

The main object is to'provide frequency discriminating apparatus of simplified or improved construction which produces a transient voltage in response to a sudden change in the phase of the reference voltage, which transient voltage is in phase with the steady state output voltage of the apparatus.

Another object is to provide frequency discriminating apparatus which may be expeditiously adjusted to precise balance during manufacture.

A frequency discriminating network of the type prises a Wheatstone bridge in which one arm is a resonant circuit tuned to a certain frequency and in' which the other three arms are equal resistors. When an alternating current of the cervalue of each of the other three bridge arms so that no output alternating current appears at the bridge output terminals. When an alternating current'of a frequency below the certain frequency is applied to the bridge input terminals, the resonant arm becomes effectively an inductance so that an alternating current is produced at the bridge output terminals in such sense as to lag in phase with respect to the input alternating current. When an alternating current of a frequency above the certain frequency is applied to the bridge input terminals. the resonant arm becomes effectively a capacitor so that an alternating current is produced at the bridge output `terminals in such sense as to lead in phase with respect to the input alternating current.

The bridge network forming the subject-matter of this invention is of a type which will produce a mentioned in my copending application, Serial No. 466,509, relating to a speed control system and filedNovember 21, 1942. Hence, the cophasal transient and steady state output voltages produce a resultant output voltage which'serves to regulate motor speed as explained in my copending application, supra.

Afeature of the invention is that the frequency discriminating bridge network provides substantially maximum sensitivitywith substantially reduced bulk and weight. Both the latter are achieved by an inductance having a molybdenum permalloy dust core which is relatively small in both weight and size, such inductance occupying ai volume in one case, by way of example, ofthe order of one cubic inch with correspondingly reduced weight. Anotherfeature relates to compensation for bridge unbalance occasioned by variations-in the direct current resistance of the resonant arm due to variations in ambient temperature. A further feature involves attenuatlng mentioned above as known to the prior art com. I6 harmonics in the reference voltage. Still another indu'ctively coupled by having individual windings applied to a common core. which provides an output which is substantially twice the output of a bridge embodying fourI physical arms, including the transient output. An important advantage of the bridge of three physical arms, aside. from the doubled output, is that balance during manufacture may be expeditiously obtained by adjusting the number of turns in the winding of one of bodying three physical arms, two of which are the two arms coupled inductively on the common COTS.

the following description taken together with the accompanying drawings in which:

The invention will be readily understood fromv Fig. 1A is a schematic circuit diagram showing a specic embodiment of the invention in a four-v arm Wheatstone bridge;

Figs. 1B, 1C. and 1D are schematic circuit diagrams showing Fig. 1A in simplified forms in response to certain action therein;

Figs. 2 and 3. 'are curves illustrating certain action invFigs. 1A, 8,. 9, 10 and 11;

Figs. 4, 5, 6 and I are vectorial representations of certain action in Figs. 1A, 8, 9, 10 and 11;

Fig. 8 is a schematic circuit diagram showing a modification of Fig. 1A;

Fig. 9 is a schematic circuit diagram showing a modification of Fig. 8;

Fig. l0' is a schematic circuit diagram of a ther modification of Fig. 1A; and

Fig. 11 is a schematic circuit diagram showing a modification of Fig. l0.

The same reference numerals are employed to designate identical elements appearing in the several figures of the drawings.

Referring to Fig.` 1A, awheatstone bridge III comprises input terminals I and 4 across which fur..

is applied a source, notshown, of an electrical I maining three arms.

A qualitative analysis of the operation of Fig. i

1A shows that'when an electrical voltage of the certain frequency fn is applied to the input terminals I and 4, the resonant arm I4 possesses effectively a resistance value equal to that of each of individual resistors I5. As the amounts of relsistance in all four arms of the bridge 'I0 are now equal, the bridge I0v is effectively balanced from an electrical standpoint so that no output voltage is produced across the output terminals 2 and 3. In this condition, the bridge of Fig. lA

output terminals 2 and 3 in such sense as to lag in 75 which is 1, and

phase by degrees the voltage applied to the i input terminals I and 4.

When the frequency of the voltage applied to the input terminals I and 4 is slightly higher than the certain frequency fo, the resonant arm possesses a capacitive component I8, as the current of the capacitor I3 exceeds the current of the winding I I. Now, the bridge of Fig. 1A may be simplified in an electrical sense as illustrated in Fig. 1D.

Hence, a small amount of voltage is produced l -across the output terminals 2 and 3 in such sense as toA lead in phase by 90 degrees the voltage applied to the input terminals I and 4. Asthe inphase components of voltage balance out in Fig. 1A, a small resultant electromotive force occurs across the bridge output terminals 2 and 3 in such sense as to lag or lead by 90 degrees the phase of the voltage applied to the bridge input terminals I and 4. This action of Figs. 1B, 1C and lDis illustrated in the curves of Figs. 2 and 3;

and is further represented by the vectors of Figs.

4, 5 and 6, respectively.

A quantitative analysis 1A is as follows:

Let E1-4=E sin wt Let o=21rf, where l is the frequency of the voltof the operation` of Fig.

age applied to the input terminals 1-4;

l QoL-m and w2LC=1, where L is' the inductance of the winding II;

Let r be the series effective resistance o! the v winding II;

Let q be the ratio is thus negligible. y

For some frequency -=u+Aw where A is small so that the phase angle but not the magnitude of the current through the lower left arm of the bridgein Fig. 1A changes, it can be readilyv shownthat the v oltage existing across terminals 2, 3 of the bridge is EFFE: cos we (1) This expression represents the steady state condition corresponding to a slight departure from the frequency at which the bridge is balanced.

If there is a sudden shift in phase of the voltage applied to the input terminals I, 4 of the bridge, a transient voltage will also appear across bridge terminals 2, .ldue to the stored energy in the resonant arm I4. This may arise from a sudden change in mechanical load or line voltage, for example. Such transient will have a magnitude dependent upon the time in which the sudden phase shift occurs, and its magnitude will be greater as the time is shorter, land will die out exponentially in a very short time. The general form of expression for the transient current is:

bn wot-g cos 100+ J- Lt cos wot-I-Be L sin wat t .4e L (2) To determine the constants A and B it is necesnut ,itqislargewecanput assume sary to consider the physical conditions of the circuit. Let it be assumed that, when t=o. the

Y phase oi the voltage impressed on the bridge input terminals I and l is instantaneously shifted by an angle qu, due to a sudden mechanical load being clutched to the motor which was previously identied and whose speed is being regulated according to my copending application, supra.

Then, the prior value of iz. was

Now, we know that the tuned` circuit Il in Fig. 1A contains stored energy 1/2LL2-}-1/2Cl.'z which energy takes a iinite time to change when a limited amount of power which has to pass through the lower` left resistance arm l5 is supplied thereto. We are therefore justiiied in assuming that at the instant t=o when the phase shift p is made and also for a quarter cycle later when 4 Z; that the current iz. remains substantially at tire old value given by Equation 3.

Substituting t=o in Equation 3 gives {wb2-15min q cos it) Putting this value for iz. in Equation 2 and putting .t=o, gives Y T i tra; y

the value of iz. from 'Equations 3 and 2 gives It will be noted that if qi Vis zero, then A and B become zero and there is no transient term in Equation 2. However, if p is small but not zero,

then

. 1 Aar-2% sin and B-g-gsin e The ratio oi l 1, maqm. w

Therefore, A maybe neglected relative toB, and

o6 which makes l sind:

and Equation 2 becomes ,7o Flusn iff-q cos w+ The voltage lib-4 across the inductance His diz, "wtlfa In order to derive the bridge output voltage Ez-a in the proper polarity it is necessary to follow exactly the same procedure used in deriving the steady state voltage as per Equation l, i. e. we take Ez-s as Ei-s-Ei-z. But E1-a=Ei-4Ez4 Substituting the values for i1. and 4 (El. Y

dt from Equation 4 the value oi 15h-:s will simplify to Bin un (When making these substitutions put R=q2r and q-1z=0 This result when compared with that of Equation 1 yields the following conclusion which has important practical value, i. e.: The bridge in Fig. 1A yields a transient voltage at the bridge output terminals 2-3, which transient voltage is su-bstantially in-phase with the steady state bridge y phase angle qu is positive so that Equations 1 and 5 are cumulative. It will also be noted that the transient voltage in Equation 5 hasla :relatively large initial value which assists stabilization by overcoming time la'gs elsewhere in the speed regulator system according to my copending application, supra.

For example, suppose the phase angle p is 25 degrees; then from Equation 5 the initial value oi the transient voltage will be .2lEo. A tenth oi one per cent change in `speed on a steady state basis from Equation 1 with a coil having a q=20,

gives .02Eo. In other words, the initial value oi the transient effect is ten times that of the steady state eiIect for the conditions, assumed. The

transient however lasts only =a few millisecondsI since IQ-20o in the specinc case supposed above. This is desirable as it provides a large initial corrective force for any change affecting the phase of the applied bridge voltage.' Due to its short duration, there is no tendency to overcorrection and the result is both fast and stable regulation of the motor speed according to my copending Vapplication, supra.

The Equation 5 for the transient output volt` mmals 2 and 4 and the voltage En. If now the input voltage E14, end ,with it the voltage E24,

Y were suddenly shifted back by an angle p to the fullA line position shown in Fig. '7 while the voltage En. due toenergy' storagein the inductance Il and capacitor I3, as above explained, does not move with it for several cycles. as indicated by its full line position in Fig. 7, then a resultant Ea-a will appear at the output terminals 2-3 and persist thereat a few cycles in accordance with Equation 5, the phase of the voltage En finally resuming its ultimate position of 180 degreesfrom E11-4 or Eo as shown by its broken line position in Fig. '7. It will be noted that the resultant vector E:3, Fig. 7, agrees with Equation 5. The initial value of Ez-a is equal to Ein-4 sin o, and the phase of Ez-z is at right angles to E1-4.4 A sudden decrease in the motor load would cause the occurrence of opposite speed and transient effects.

The foregoing analysis of the circuit of Fig; 1A with respect to both steady state and transient conditions, and the quantitative results shown by the calculations indicate advantageous perform- .ance to be sought in a proper and suitable embodiment. securing these advantageain accordance with the present inventionl will now be described. Substantially maximum sensitivity with substantially maximum-minimum bulk and weight is achieved by utilizing in the resonant arm l 4 aninductance having a core l2 of molybdenum permalloy dust4 One form of such embodiment for Ii120 per cent of the equilibrium frequency lo.

-It is also apparent from Equation 1 that the applied bridge voltage, Fig. 1A, should be a maximum consistent with the flux density limits of the inductance comprising the winding Il on the core I2, and the power consumption of the circuit. Since amplification by vacuum tubes is relatively inexpensive, the limitation on bridge output is xed by the ratio of the input voltage to extra- -neous voltages which may be due to""`imperfect bridge balance, or harmonics present in the input voltage, or harmonics generated in the inductance by magnetic saturation.

Consider rst th aspect of bridge balance in Fig. 1A in which the effect of ambient tempera ture variations on the bridge I0' is such as to vary the effective resistance value of the winding Il due to changes in the effective direct current resistance value of the inductance constituting the resonant arm I 4. In effect, the winding il may be deemed to possess a positive temperature coefficient of resistance. This varies the effective parallel resistance of the resonant arm i4, and. -consequently, the bridge I0 of Fig. 1A can be accurately balanced at the certain frequency fo only over a limited temperature range. Compensation for such resistance variation is accomplished by introducing a thermistor in the lower left`arm of the bridge 2l as shown in Fig. 8. The lower left arm in Fig. 8 now comprises a resistor 26 disposedin series with a resistor 2l and the thermistorn in parallel. Th'e thermistor 23 has a preselected negative temperature coemcient of resistance to vary the eective resistance of the lower left arm in a sense such that the effect of the positive temperature coefficient of v resistance of the winding of I I is compensated for In one' example, suchan inductance occupied only about one cubic inch of space with correspondingly small weight. This ,inductance possesses a satisfactory q of the order of 20 at a frequency oi'A 720 cycles per second, which for this illustration is the frequency to which the resonant arm, Fig.

1A, is tuned, due to' its high magnetic permeability. Such inductance is also stable as to its inductive characteristic with respect to temperature variations, as the core I2 is. treated substantially as disclosed in the patent of V. E. Legg, No. 2,158,132,

granted May 16, 1939.

In designing an inductance comprising the winding Il on the core I2, Fig. 1A, there is frequently an optimum value of q for any given set of conditions. Although Equation 1 shows that the bridge output, Fig. 1A, is increased the higher the value of q, there is a limit in cases involving speed control circuits of thetype disclosed in my copending application, supra, beyond which limit it is undesirable to go. Referring to the wide range frequency characteristics, Figs. 2 and 3, there is a limited frequency band width within which the proper phaserelatlons for the output voltage Ez-a will hold. If the value of q were too high, this band width may be so narrow that the motor would notmiittain a speed corresponding to the equilibrium frequency, fo, but would either stop accelerating before the speed corresponding to such equilibrium frequency fo were reached, or run through the speed corresponding to such equilibrium frequency fn. It was found that the value of q should be such that the bridge I0, Fig.

over a certain range of variation of ambient tern-` perature. This means that the bridge arm il of Fig.` 1A remains substantially balanced at the frequency fo over the certainrrange of ambient temperature variation. The thermistor 2l is preferaly located adjacent the winding Il so that both thereof are continuously exposed substantially to the same range of variations of ambient temperature. In other respects. the bridge 24 of Fig. 8 functions substantially'identically with theI bridge I0 of Fig. 1A.

Considering next the aspect of bridge balance in Fig. 1A in which the presence of a small percentage of harmonics of the voltage applied to the input voltage E1-4 would cause the occurrence of an output voltage Ear- 3, even though the linput voltage Ei4 contained the frequency fo. and therefore theoretically the bridge lll should be substantially balanced. For example. if the voltage E1.4 has 5 per cent of such harmonics, these harmonics would appear only slightly attenuated in the output voltage Ez-a, as capacitor i3 would oder small impedance to such harmonics, and the bridge I 0 would be far 01T balance for harmonies. On the other hand, for the frequency fo to which the resonant arm l5 is tuned, the bridge I0 would be'substantially balanced so that if l E23==j.02E1-4 If the magnitude of the harmonic voltage at .plied to the bridge plied rto the core i2 and connected in l posed in parallel. with a capacitor between the coupled windings the output terminals i. and 3 is half of that at the input terminals i and 4, these harmonics will be" 1/2 of 5%=.025E1-4, or a little greater than the useful signal at the output terminals 2V and 3, Fig. 1A. The presence of such large amounts of harmonics in the output of the bridge I0, Fig. 1A, will usually interfere with proper perfomance of a circuit which is to be regulated exclusively in response to variations in the fre-- quency of the input voltage Ei-4 with reference to the frequency feas pointed out in my copending application, supra.

To control the magnitude of the aforementioned harmonics present in the input voltage E1-4, a illter comprising resistors 28 and 28 and capacitors 30 and 3|, all of which are made of physically small and inexpensiveunits, is apinput terminals IV and 4 and bridge output terminals 2and 3 as indicated in Fig. 8. Thus, the resistor 28 is in series-with terminal 4; resistor 29is of both these terminals and capacitor 3l is across output terminals 2 and 3. In this connection the input voltage E1-4 is supplied to input terminals li and 3. This.

lter serves two important purposes: (l) to attenuate harmonicspresent in the'voltage, EH, one stage being providedby theV capacitor 30 together with resistors 28 and"-`=28,. and another stage `being-provided by capacitor 3l', operating in series with terminal ,I, capacitor 30 is in shunt against the internal resistance of the bridge24;` 'Y

and (2) to provides. compensating phase shift of approximately 90 degrees between the input and output voltages of the bridge 24 so as to-offset the approximate 90 degree internal shift inherent in the bridge 24 as shown by Equation By such lter means Ythe bridge output voltage En-a is rendered in-phase with the voltage applied to the input terminals 5 and 6. This phase characteristic, Fig. 8, and the compensationtherefor are important when it is desiredl to translate the alternating output voltageEz-a into a direct current voltage in the manner pointed out in my copending application, supra.

Anciier embodiment of the invention is shown in Figs-9 in which approximately twice the output voltage Ez-s of Figs. 1A or 8 is obtained; In an electrical sense are identical as it will now be pointed out. It will be evident from Fig. 1A that if an additional tuned circuit, exactly like the tuned arm I4, were substituted in the lowerrig-ht arm of FigflA that the bridge output for a -given frequency change would be substantially doubled. In this connection Fig. 10 shows, in the upper left arm of a Wheatstone bridge 35, the winding apparallel with the capacitor arm I4 shown and described above concerning Fig. 1A. In the lower right arm of the bridge 33, Fig. 10, is interposed a resonant arm 3B comprising winding 3,1 applied to corey 38 and-dis- 39. As the resonant arms |4 and 36 are substantially symmetrical, the core 38 may be omitted from the resonant arm 38 ,and the winding 31 thereon applied to the core i2 of the resonant arm I4. At the same time, the upper `right hand resistance arm IB, Fig. l0, and the capacitor 38 of the resonant arm 36, Fig. 10, may be both omitted as the electrical equivalents of these elements will be effectively provided by the transformer action II and 31 on the common core I2. This results in bridge 4|I shown in Fig. 11, which latter bridge is the electrical the devices of Figs. 8 and 9- pacitance of I3 to constitute the resonant age at the bridge lto said first winding. in a The coil embodying the windings and 31'applied to the common core l2, Fig. 9, is not appreciably larger in size than the coil comprising only the individual `winding on the core l2, Fig. 1A, as the since it carries no current.`

An important feature of the bridges 43 and 4| shown in Figs. 9 and lll is that balance may be expeditiously obtained in the lshop during` manufacture by adjusting but one side of the bridges 40 and 4|, whereas in the case ofthe bridges |0, 3| and 35, Figs. 1A, 8 and l0, respectively, order to effect bridge balance. The adjustment procedure for balancing Figs. 9 and 11 is to vary the number :i turns of winding on core. I2 until the 'arm |4 is tuned precisely to the frequency fo so that the arm I4 possesses a predetermined effective impedance at the latter frequency. Then, impressed on bridge input terminals and 4, a variably calibrated resistor is substituted in the lowerleft arm |-3, and adjusted until. the voltoutput terminals 2 and 3 is substantially zero. The calibrated resistor is then replaced by a permanent thermlstor network of substantially the same resistance value at the ambient temperature at which the adjustable re sistor was used. This means that the effective impedance o: the lower left arm I-3 at the frequency f5 approximates the effective impedance frequency fo. This method of adjustment permits liberal-manufacturing tolerances with reference to the values of the cathe capacitor` I3 and number of turns of the winding Il, and further gives precise over-all bridge balance. It should be noted that no exact turns ratio between windings and 31 is required; since a variation of the turns of the winding 31, for example. merely alters the arm, I-3'. A

What is claimed is: 1. A Wheatstone bridge an electrical winding, a capacitor, and a magnetic core for said winding, said nrst arm being tuned to a certain frequency, in a second arm a resistive network, in a third arm a further electriand coupled inductively fourtharm the elective cal winding on saldcore mutual inductance between said windings on said core, a pair or input terminals constituted by a terminal of said first arm andthe junction point between said second and third arms, :and a pair of output terminals constituted'by the other end of I said third arm and lsaid first the junction point between and second arms, balanced condition when electrical energy having the certain frequency is applied to said input terminal pair so that substantially none of the applied input energy appears at said output terminal pair, and having an unbalanced condition when n a frequency diiferent from the certain frequency is supplied to said Ainelectrical energy having put terminal pair so that some of the latter energy appears at said output terminal pair.

2. A bridge according to claim'l in which .said

v 5 lo, rig. 1A. rms undue an winding 31 may be of une` wire all arms must be precisely balanced in with the voltage of frequency .fo

having in-anrstarrn' `v said bridge having e 7 winding of said rst arm tends to change its effective resistance with variations in ambient i temperature over a certain range and thereby tends to unbalance said bridge although electrical energy of the certain frequency is applied to said 'input terminal pair, and in which said resistive network of said second arm includes a resistive. element having a preselected temperature coeilicient of resistance of ysuch magnitude and sign that the effective resistance of said second arm tends to compensate for the changes in the efiective resistance of said flrst arm during the certain ambientV temperature variation and thereby tends to maintainthe balanced condition in said bridge. Y

v 3. 'I'he apparatus according to claim 1 in which said core is composed of molybdenum-permalloy dust so that said apparatus possesses'substan-A tially reduced bulk and weight.

4. A bridge according to claim 1, said bridge possessing an inherent phase'characteristictending to introduce a phase shift in the electrical energy supplied to said output terminal pair when the bridge is unbalanced. said bridge including a aseaeis frequency-selective network applied effectively to both saidinput and output terminal pairs and cooperating with the effective internal resistance of said bridgefor substantially attenuating harmonics that may be present in the electrical energy applied to said input terminal pair, said network having a phase characteristic such as to compensate for the phase characteristic of said bridge.

5. A frequency prising-a Wheatstone bridge havinga pair of input terminals and a pair of output terminals, a resonant arm comprising an inductance and capacity connected between an input terminal and an output, terminal. a resistance arm connected between said output terminal and the other input terminal, and aninductance arm connected between said last-mentioned input terminal and the remaining output terminal and closely coupled inductively to said first-mentioned inductance, the remaining arm of said bridge comprising the mutual inductance between said last'two vcoupled inductances.

' HUGH S'I'OIIER.

discriminating networl: vcom-k i 

